The invention relates to the design and implementation of wireless rings and network of links in millimeter wave (MMW) line of sight wireless communications.
It is normally necessary to obtain a xe2x80x9cspectrum licensexe2x80x9d before using a frequency band for wireless communication. This is true of millimeter wave (MMW) frequencies, as it is for most other frequency bands. Spectrum licenses for MMW communications frequencies are issued by agencies responsible for management of the electromagnetic spectrum, and generally permit the use of two frequency bands for full-duplex (bi-directional, concurrent) communicationsxe2x80x94one band for transmitting (Tx) and one band for receiving (Rx).
FIG. 1 is a graphical representation of a typical single spectrum license 100 for full-duplex MMW communication comprising a first frequency band 102 and a second frequency band 104. The first frequency band 102 (xe2x80x9cChannel 1xe2x80x9d) centers about a first frequency f1 and has a first bandwidth xcex94f1. The second frequency band 104 (xe2x80x9cChannel 2xe2x80x9d) centers about a second frequency f2 and has a second bandwidth xcex94f2. The second frequency f2 is distinct and offset from the first frequency f1. The second bandwidth xcex94f2 is typically substantially equal to the first bandwidth xcex94f1. In the context of the present invention, the first frequency band 102 is used for communication in a first direction between two xe2x80x9cnodesxe2x80x9d of a MMW communication network, and is generally indicated by a solid line with a left-pointing arrowhead, and the second frequency band 104 is used for communication in a second direction between the two nodes, and is generally indicated by a dashed line with a right-pointing arrowhead When referring to spectrum licenses in subsequent figures, dashed and solid lines will be used to represent the two distinct frequencies of a spectrum license, with arrowheads indicating the direction of communication, from transmitter to receiver. A bi-directional communication path between two adjacent nodes communicating over two distinct frequency bands is referred to as a xe2x80x9clink,xe2x80x9d and each link uses one spectrum license.
FIG. 2 is a block diagram of a multi-node MMW communication network 200 comprising a xe2x80x9cchainxe2x80x9d (or xe2x80x9cringxe2x80x9d) of communication nodes 202, 204, 206, 208, 210, 212, and 214. The system 200 employs multiple frequency licenses between multiple nodes.
The first node 202 (#1) communicates with the second node 204 (#2) over a link 216 via a first frequency band 216a and a second frequency band 216b. The first frequency band 216a is used for communications from the second node 204 to the first node 202 and the second frequency band 216b is used for communications from the first node 202 to the second node 204.
The first node 202 (#1) communicates with the third node 206 (#3) over a link 218 via a first frequency band 218a and a second frequency band 218b. The first frequency band 218a is used for communications from the third node 206 to the first node 202 and the second frequency band 218b is used for communications from the first node 202 to the third node 206.
The second node 204 (#2) communicates with the fourth node 208 (#4) over a link 220 via a first frequency band 220a and a second frequency band 220b. The first frequency band 220a is used for communications from the second node 204 to the fourth node 208 and the second frequency band 220b is used for communications from the fourth node 208 to the second node 204.
The third node 206 (#3) communicates with the fifth node 210 (#5) over a link 222 via a first frequency band 222a and a second frequency band 222b. The first frequency band 222a is used for communications from the third node 206 to the fifth node 210 and the second frequency band 222b is used for communications from the fifth node 210 to the third node 206.
The fourth node 208 (#4) communicates with the sixth node 212 (#6) over a link 224 via a first frequency band 224a and a second frequency band 224b. The first frequency band 224a is used for communications from the sixth node 212 to the fourth node 208 and the second frequency band 224b is used for communications from the fourth node 208 to the sixth node 212.
The fifth node 210 (#5) communicates with the seventh node 214 (#7) over a link 226 via a first frequency band 226a and a second frequency band 226b. The first frequency band 226a is used for communications from the seventh node 214 to the fifth node 210 and the second frequency band 226b is used for communications from the fifth node 210 to the seventh node 214.
Any ring/chain of xe2x80x9cnxe2x80x9d nodes can be implemented with at most xe2x80x9cnxe2x80x9d spectrum licenses, but in practice the required number of licenses is usually considerably smaller in point-to-point, line of sight MMW communication. In the system 200 of FIG. 2, some of the links 216, 218, 220, 222, 224 and 226 can use common spectrum licenses. The actual number of spectrum licenses used to implement a network of nodes is selected based upon the geometry of the network, the directionality of antennas, the local geometry of each node site and antenna polarization employed at the nodes. In combination, these parameters determine the amount of interference between nodes employing the same spectrum license. MMW receiving systems are frequency-selective enough that xe2x80x9ccross-bandxe2x80x9d interference (interference between transmissions from different frequency bands) is negligible.
For example, if the antennas for the second node 204 (#2) and the fourth node 208 (#4) exhibit a high degree of electromagnetic separation from those of the third node 206 (#3) and the fifth node 210 (#5), then the links 220 and 222 can employ the same spectrum license. Because of the short wavelength of MMW frequencies, MMW antennas are inherently highly directional. As a result, a high degree of electromagnetic separation between nodes can be achieved in MMW when the antennas for those nodes are not aimed at one another. In some cases, the combination of antenna directionality and physical separation between independently communicating pairs of nodes ensures negligible interference therebetween, permitting re-use of a single spectrum license.
The basic building block of a multi-node point-to-point MMW network is a serial chain of three nodes, communicating using two links (which may or may use the same spectrum license, as discussed above) between a middle node and two adjacent nodes on opposite sides of the middle node. For example, the first node 202, second node 204 and third node 206 of FIG. 2 form such a chain (A chain of two nodes using only one spectrum license is a trivial case and does not account for the effects of inter-node interference that exists in longer chains of nodes. A three-node chain is the minimum size network subset that exhibits these interference effects.)
In a typical serial chain of three nodes, the two adjacent links often employ two different spectrum licenses. A necessary (but not sufficient) condition for the use of a common spectrum license is that the xe2x80x9cmiddlexe2x80x9d (intermediate) node of the three node chain transmits to both adjacent nodes using one frequency band (channel) of a spectrum license, and receives transmissions from both adjacent nodes via the other frequency band of the spectrum license. For example, consider a case where all of the links 216, 218, 220, 222, 224 and 226 of FIG. 2 employ the same spectrum license. In this case, all of the first frequency bands 216a, 218a, 220a, 222a, 224a and 226a would be the same and all of the second frequency bands 216b, 218b, 220b, 222b, and 224b would be the same. Node #1 202 transmits to nodes #2 and #3 (204 and 206) on the second frequency band (216b, 218b) and receives from nodes #2 and #3 (204 and 206) on the first frequency band (216a, 218a). Node #2 (204) transmits to nodes #1 and #4 (202 and 208) on the first frequency band (216a, 220a) and receives from nodes #1 and #4 (202 and 208) on the second frequency band (216b, 220b). Node #3 (206) transmits to nodes #1 and #5 (202 and 210) on the first frequency band (218a, 222a) and receives from nodes #1 and #5 (202 and 210) on the second frequency band (218b, 222b). Node #4 208 transmits to nodes #2 and #6 (204 and 212) on the second frequency band (220b, 224b) and receives from nodes #2 and #6 (204 and 212) on the first frequency band (220a, 224a). Node #5 210 transmits to nodes #3 and #7 (206 and 214) on the second frequency band (222b, 226b) and receives from nodes #3 and #7 (206 and 214) on the first frequency band (222a, 226a). In the case of each node, transmission to both adjacent nodes occurs in one frequency band and reception from both adjacent nodes occurs in the other frequency band. Because of this, and because of the frequency selectivity of MMW receivers, transmissions at any node do not interfere in any significant way with reception. This property is referred to hereinafter as xe2x80x9ctransmit-receive separationxe2x80x9d.
In the case described above where all of the links (216, 218, 220, 222, 224 and 226) employ the same spectrum license, transmit-receive separation prevents interference within a node due to its own transmissions. Attention is now directed to interference from other nodes, using node #1 202 as the point of reference.
Node #1 does not experience interference from node #4 208 or node #5 210, because both nodes transmit using the second frequency band (220b, 222b) while node #1 202 receives only on the first frequency band (216a, 218a). Frequency selectivity limits cross-band interference to negligible levels. Since appreciable sources of interference with node #1 202 can only occur in the first frequency band, the only possible remaining sources of interference come from node #2 204, node #3 206, node #6 212 and node #7 214, which transmit on the first frequency band (216a, 218a, 224a and 226a).
In a chain of nodes, nodes adjacent to one another are referred to herein as xe2x80x9cfirst neighborsxe2x80x9d, nodes separated in the chain by another node are referred to as xe2x80x9csecond neighborsxe2x80x9d, nodes separated in the chain by two other nodes are referred to as xe2x80x9cthird neighborsxe2x80x9d, and so on. Accordingly, node #1 202 and node #2 204 are first neighbors, node #1 202 and node #4 208 are second neighbors, and node #1 202 and node #6 212 are third neighbors.
Evidently, the only significant sources of interference in a chain of nodes of this type are from odd-numbered neighbors, i.e., first neighbors, third neighbors, etc. However, in any likely network configuration, third neighbors (and those beyond xe2x80x9cthirdxe2x80x9d) are likely to be aimed considerably off-angle with respect to one another. That is, third neighbors are not aimed at one another, and are generally aimed quite far away from one another. Due to the high degree of directionality of MMW antennas this contribute to a high degree of angle separation between third (and higher numbered) neighbors. In typical MMW networks, this angle separation translates to signal attenuation (signal separation) of 60 dB, or better. Therefore, the main sources of interference are adjacent nodes (first neighbors). Referring to the aforementioned three-node building block, all appreciable interference at any node resides completely within the building block that includes its two first neighbors. In the case of node #1 202, all appreciable interference occurs from its two first neighbors, node #2 204 and node #3 206.
In the case where the two links of a three-node building block use two different spectrum licenses, there is inherently a sufficient electromagnetic separation (frequency selectivity) to prevent cross-band interference. FIG. 3 is a block diagram of a three-node building block 300, comprising three nodes, a first node 302 (#1) having a first antenna 302a and a second antenna 302b, a second node 304 (#2) and a third node 306 (#3). The first antenna 302a communicates (full duplex, bi-directionally) with the third node 306 via a link 318 comprising a first frequency band 318a and a second frequency band 318b. The first antenna 302a receives transmissions from the third node 306 via the first frequency band 318a and transmits to the third node 306 via the second frequency band 318b. The second antenna 302b communicates (full duplex, bi-directionally) with the second node 304 via a link 316 comprising a first frequency band 316a and a second frequency band 316b. The second antenna 302b receives transmissions from the second node 304 via the first frequency band 316a and transmits to the second node 304 via the second frequency band 316b. In the case illustrated in FIG. 3, the first link 316 and the second link 318 use different spectrum licenses, ensuring that electromagnetic separation (frequency selectivity) reduces interference at either the first antenna 302a or the second antenna 302b of the first node to negligible (acceptable) levels. Accordingly, the four receive and transmit paths associated with the four frequency bands 316a, 316b, 318a, and 318b used by links 316 and 318 can be viewed as four completely independent channels.
In the case where the two links of a three-node building block use the same spectrum license, frequency re-use is doubled, but there is greater potential for interference.
FIG. 4 is a block diagram of another three-node building block 400, comprising three nodes, a first node 402 (#1) having a first antenna 402a and a second antenna 402b, a second node 404 (#2) and a third node 406 (#3). ). The first antenna 402a communicates (full duplex, bi-directionally) with the third node 406 via a link 418 comprising a first frequency band 418a and a second frequency band 418b. The first antenna 402a receives transmissions from the third node 406 via the first frequency band 418a and transmits to the third node 406 via the second frequency band 418b. The second antenna 402b communicates (full duplex, bi-directionally) with the second node 404 via a link 416 comprising a first frequency band 416a and a second frequency band 416b. The second antenna 402b receives transmissions from the second node 404 via the first frequency band 416a and transmits to the second node 404 via the second frequency band 416b. Unlike the links 316 and 318 of the building block 300 of FIG. 3, which use different spectrum licenses, the two links 416 and 418 use the same spectrum license. As a result, the two first frequency bands 416a and 418a are the same and the two second frequency bands 416b and 418b are the same. As a result, the separation between the desired receive signal in the first antenna 402a (in the first frequency band 418a) which comes from the third node 406 and an undesired or xe2x80x9cinterferingxe2x80x9d signal from the second node 404 (in the same frequency band 416a) is due only to the angular separation between the two antenna 402a and 402b. Since transmissions from node #2 404 to the first node are aimed directly at the second antenna 402b which is closely co-located with the first antenna 402a, the aiming of transmissions from node #2 does not contribute to this angle separation, and a portion 430b of the signal transmitted from node #2 404 to the second antenna 402b of the first node xe2x80x9cleaks intoxe2x80x9d the first antenna 402a. In a typical point-to-point MMW communications system as described herein with respect to FIG. 4, the ratio of an interfering signal from an adjacent node to a desired signal can reach as high as xe2x88x9230 dB. The same situation holds in reverse for the second antenna 402b. That is, a portion 430a of the signal transmitted from node #3 406 to the first antenna 402a of the first node xe2x80x9cleaks intoxe2x80x9d the second antenna 402b. 
Depending upon site geometry, node locations and the angle separation between the antennas, interference when using a single spectrum license as described hereinabove with respect to FIG. 4 can be too great for reliable MMW communication. When this occurs, it becomes necessary to resort to using two separate spectrum licenses, reducing frequency re-use and increasing cost.
It should be understood that different modulation schemes can be implemented in MMW communications including, but not limited to, Quadrature Amplitude Modulation (QAM), and Pulse Position Modulation (PPM).
Receive interference and transmit interference are problems which can occur when two or more nodes employ the same frequency license.
FIG. 4 illustrates a typical example of xe2x80x9creceive interferencexe2x80x9d. In this example, a signal transmitted to node #1 402 from node #2 404, which is intended for antenna 402b of node #1 402, leaks to antenna 402a of node #1 402. Similarly, antenna 402b receives the leakage of signals transmitted from node #3 406, which are intended for antenna 402a. These signals are referred to as xe2x80x9creceive interfering signalsxe2x80x9d. The interference power depends on the geometry of the node site and on the receive pattern of the antennas 402a and 402b. If the antennas 402a and 402b are located exactly back to back (180 degrees separation), the leakage of interfering signal is of the order of xe2x88x9260 to xe2x88x9240 dB (for MMW high directivity antennas). However, if the angle is not 180 degrees, stronger leakage may arise, and may reach the order of xe2x88x9225 dB, depending on the local geometry.
For purposes of the discussion that follows, it is assumed that the receive interference at a given antenna of a given node, is solely a result of transmission made from an adjacent node, which is intended for the other antenna at the given node. This would result from the two links to the two nodes adjacent the given node employing the same frequency license.
The amount of receive interference depends on whether or not ATPC is used, and whether or not the stations (nodes) employ different modulation schemes. Channel attenuation due to weather effects (i.e., rain attenuation, or CPA in dually polarized systems) must also be taken into account The following cases (A-D) are therefore possible.
A) Without ATPC, Same Modulation for all links: Link lengths may not be constant along the chain/ring, depending on the physical locations of the nodes. Moreover, according to weather conditions, one link can suffer from high attenuation due to rain, while the other can see clear sky. Typical accumulated rain attenuation value in MMW links can reach 30 dB in bad weather conditions. Thus the dynamic range of receive interference to desired receive signal can be very high, from the range of xe2x88x9260 to xe2x88x9240 dB without rain, to the range of xe2x88x9230 to xe2x88x9210 dB, depending on angles between antennas and weather conditions.
B) With ATPC, Same Modulation for all links:xe2x80x94Typical values of ATPC in MMW systems is 10 to 25 dB dynamic range. With 20 dB ATPC, the receive interference may be reduced to xe2x88x9220 dB, or worse.
C) Without ATPC, Different Modulations along the chain/ring: Employing different modulations along the chain/ring further increases the dynamic range of receive interference to desired signal. In the absence of ATPC, each antenna transmits with the maximal possible power. When different modulations are employed, the maximal power varies according to the required back-off. The difference in back-off required for 16 QAM and 128 QAM is in the order of 5 dB (this takes into account not only the different peak-to-r.m.s. of the constellations, but also the sensitivity to impairments). Consider, for example, the following. Assume, for example, that the link 416 from node #2 404 to node #1 402 in FIG. 4 employs 16 QAM (or 16 TCM), while the link 418 from node #3 406 to node #1 402 employs 128 QAM (or 128 TCM). This enhances the receive interference at antenna 402a by an amount of 4 dB, due to the fact that the back-off required by the transmitter in node #3 406 is larger by 5 dB than that required by the transmitter in node #2 404. Since ATPC is not present, the fact that decoding 16 QAM needs lower received power cannot be taken into account. Thus, the conclusion is, as in the case of fixed modulations among the three stations, when no ATPC is present, the ratio between receive interference and desired signal can exceed considerably the value of xe2x88x9230 dB.
D) With ATPC, Different Modulations alone the chain/ring: In this case, the receive interference is increased relative to that in case B (with ATPC, same modulation) by an amount equal to the difference in sensitivity. The difference in sensitivity between 16 QAM and 128 QAM may typically be 9 dB. Thus, the interference may be roughly only 10 dB below the desired signal.
FIG. 5 is a block diagram of a three-node building block 500, comprising three nodes, a first node 502 (#1) having a first antenna 502a and a second antenna 502b, a second node 504 (#2) and a third node 506 (#4). The first antenna 502a communicates (full duplex, bi-directionally) with the third node 506 via a link 518 comprising a first frequency band 518a and a second frequency band 518b. The first antenna 502a receives transmissions from the third node 506 via the second frequency band 518b and transmits to the third node 506 via the first frequency band 518a. The second antenna 502b communicates (full duplex, bi-directionally) with the second node 504 via a link 516 comprising a first frequency band 516a and a second frequency band 516b. The second antenna 502b receives transmissions from the second node 504 via the second frequency band 516b and transmits to the second node 504 via the first frequency band 516a. As was the case with the links 416 and 418 of the building block 400 of FIG. 4, the two links 516 and 518 use the same spectrum license. As a result, the two first frequency bands 516a and 518a are the same and the two second frequency bands 516b and 518b are the same.
FIG. 5 illustrates a typical example of xe2x80x9ctransmit interferencexe2x80x9d. In this example, only one spectrum license is employed for the two links 516 and 518; Due to the out-of-main beam linkage, the signal transmitted from antenna 502b of node #1 502, which is intended for node #2 504, leaks also in the direction of node #3 506, as indicated by the arrow 532b, and is received at node #3 506 as interference, owing to the fact that channels 1 coincide. Similarly, the signal transmitted from antenna 502a of node #1, which is intended for node #3, leaks, as indicated by the arrow 532a, to node #2. This leakage is referred to as xe2x80x9ctransmit interference signalsxe2x80x9d.
As was the case with receive interference, the amount of transmit interference depends on whether or not ATPC is used, and whether or not the stations employ different modulation schemes. The following cases (E-H) are therefore possible:
E. Without ATPC, Same Modulation for all links: In this case, the ratio of transmit interference to desired signal is determined solely by the amount of out-of-main beam leakage (directivity) of the antenna and the neighborhood of the antenna. This is due to the fact that even in case of strong rain attenuation, the desired and interfering signals are subject to the same rain attenuation, and the constellations and transmission power are similar. Thus the transmit interference is very weak in case of good direction separation (for MMW, about xe2x88x9260 to xe2x88x9240 dB), and can deteriorate to xe2x88x9230 dB or below, depending on the angle between the links and the site geometry.
F. With ATPC, Same Modulation for all links: Typical ATPC dynamic range is 10 to 20 dB. With 20 dB ATPC, transmit interference to desired signal ratio can reach a level of xe2x88x9220 dB.
G. Without ATPC, Different Modulations alone the chain/ring: Transmit interference deteriorates from the situation of no ATPC and one modulation, by an amount equal to the differences between required back-off, which may reach values of the order of 5 dB between 16 QAM and 128 QAM for example. This results in a value of about xe2x88x9225 dB transmit interference. The maximal allowed interference at each modulation is determined by its sensitivity.
H. With ATPC, Different Modulations alone the chain/ring: In this case transmit interference may deteriorate by an amount of 5 dB relative to the case of ATPC with one modulation scheme. Thus it may reach xe2x88x9215 dB or worse.
The above-described scenarios for receive and transmit interference are summarized in the following table.
Evidently there is a need for a technique to improve frequency re-use and/or to compensate for the effects of receive and transmit interferences between nodes of a multi-node MMW network.
It is a general object of the invention to provide a technique for improving the frequency re-use of links in (i.e., improving the spatial spectral efficiency of) a wireless ring and network of links, in systems where the electromagnetic separation between antennas does not suffice to ensure low interference, such as is often the case in millimeter wave (MMW) communications.
Generally, the basic building block of a multi-node point-to-point MMW network is a serial chain of three nodes (stations), a xe2x80x9cmiddlexe2x80x9d node (#1) and two xe2x80x9cadjacentxe2x80x9d (or xe2x80x9cfirst neighborxe2x80x9d) nodes (#2, #3) on either side of the middle node, communicating using two links (link 1-2, link 1-3). When a common spectrum license is used for the two links, the xe2x80x9cmiddlexe2x80x9d node transmits to both adjacent nodes using one frequency band (channel) of the spectrum license, and receives transmissions from both adjacent nodes using the other frequency band of the spectrum license. Generally (without the present invention), interference at the intermediate node connecting the two links is relatively high, since antennas of the adjacent nodes are directed toward the intermediate node, thus there is an angle separation (directivity) due to only one antenna. Angle separation due to only one antenna may not be enough to ensure low interference.
According to the invention, a communication system (network), such as communication system is a millimeter wave (MMW) communication system, comprise a serial chain of nodes comprising a middle node (#1) a first adjacent node (#2) on a one side of the middle node and a second adjacent node (#3) on an opposite side of the middle node, each of the middle, first adjacent and second adjacent nodes comprising a transmitter and a receiver, a first antenna at the middle node (#1) for transmitting and receiving over a first link with the second adjacent node (#3), a second antenna at the middle node (#1) for transmitting and receiving over a second link with the first adjacent node (#2); and means for performing interference cancellation in one or both of the transmitter and receiver of the middle node. Various embodiments of multidimensional equalizer are disclosed for performing interference cancellation. A transmit interference canceller is disclosed comprising means for convolving a signal intended for transmission to a one adjacent node with a equalized version of a signal intended for transmission to the other adjacent node. Receive interference cancellation can be done with or without mitigating phase noise. Transmit interference cancellation can be done with or without mitigating phase noise. The invention can be used with or without employing Automatic Transmitter Power Control (ATPC) at the nodes. The invention can be used employing various modulations on links between the various nodes of the network. Techniques are disclosed for setting up the links of the network.
According to the invention, interference cancellation is performed at the transmitter only, or at the receiver only, or at both the transmitter and the receiver, depending on the specific network implementation. Interference cancellation at the receiver is done with a multi-dimensional equalizer. Interference cancellation at the transmitter is done with a multi-dimensional equalizer, and utilizes a feed back channel from the node to which the transmission is done The receive interference cancellation mitigates phase noise with the aid of a set of PLLs located on the branches of the equalizer, before or after the equalizer, or by using a common carrier reference. Transmit interference cancellation mitigates phase noise effects by using a common carrier reference.
The invention may utilize a three-stage setup process for cases where simultaneous setup of all links of the network is required. The setup is done in three stages: first, only the receive interference cancellation is activated, at the second step feedback channel is made available, and at the third step the transmit interference cancellation is activated. The setup process ensures that proper interference cancellation is achieved, although feedback channels are not initially available. The setup process requires that ATPC is turned off during setup, or the use of a smaller constellation during setup, in order to reduce the transmit interference during setup. The invention may alternatively utilize a link-by-link sequential setup process. In this case, the setup is done in three stages for each node. First receive interference cancellation is activated, at the second stage feedback channel is made available, and at the third stage transmit interference cancellation is activated.
The present invention improves frequency re-use in systems where the electromagnetic separation between antennas does not suffice to ensure low interference. It enables the use of one spectrum license in two adjacent links, by performing interference cancellation at the intermediate node. The invention is applicable to networks with stations employing single antenna, multi antenna, dual polarization, and combinations of the above. The invention is applicable to various network topologies, in particular rings and chain of links. The invention enables the implementation of a ring or chain of links employing only one spectrum license, provided that the network topology ensures that for every node, third neighbors and further does not cause significant interference. This requirement is typically met at MMW networks. The invention employs a frequency channel allocation that ensures that the transmission at each node does not interfere with reception at the same node. The invention is applicable in networks employing ATPC and various modulations along the various links.
Other objects, features and advantages of the invention will become apparent in light of the following description thereof.